A first aspect of the present invention provides a fastener installation tool for installing a fastener by gripping and drawing back a stem of the fastener, comprising a hydraulic cylinder, and a hydraulic piston assembly located in the hydraulic cylinder for use in drawing back the stem of a fastener, the hydraulic piston assembly characterised by comprising an elongate draw part of first tensile strength and a separate hollow piston part of second tensile strength, which hollow piston part is mounted on the elongate draw part such that the elongate draw part extends at least partly therethrough, wherein the elongate draw part and the hollow piston part are formed from different materials and wherein the first tensile strength is lower than the second tensile strength.
Preferably the hollow piston part provides at least part of a hydraulic fluid contacting piston surface of the hydraulic piston assembly, more preferably the hollow piston part provides substantially the entire hydraulic fluid contacting piston surface of the hydraulic piston assembly.
With the present invention, it has surprisingly been found that it is not necessary to form the hollow piston part and the elongate draw part of a fastener installation tool from the same high strength and high toughness material, and it has also been found that there are unexpected advantages associated with forming the hollow piston part and the elongate draw part from different materials, as explained below.